A fuel pump moves fuel from a car's gas tank to the engine's fuel injection system. Older cars used mechanical pumps that moved fuel into a carburettor. Today's cars have efficient, computer-controlled electronic pumps that deliver fuel to a fuel injection system.
The Audi A6 (years 1998 through 2004) has two different fuel pump and fuel-level sender (a device that tells the gas gauge how much fuel is in the tank) combinations, depending upon the model. In the Quattro models, the fuel pump is combined with one of two fuel-level senders in the fuel tank. The fuel pump and fuel-level sender are combined in the front-wheel drive models.
The fuel pump is located in the fuel tank, which is located under the back seat of the Audi A6. Remove the back seat cushion to get closer to the fuel pump (to listen for a running pump, for example). A cover plate indicates the location of the fuel pump, which is on the right side of the fuel tank for both models (driver side is left). Note, the Quattro has an additional cover plate for the second fuel-level sender on the left side of the tank.
Do not attempt to remove the fuel pump cover. This task requires a special tool and exposes a very flammable gas tank.